36

I want to sort String elements in the array months by length using Arrays.sort method. I was told here, that it's possible to use lambda expressions instead of creating new class implementing Comparator. Did it exactly the same way, yet it doesn't work.

import java.util.Arrays;
import java.util.Comparator;

public class MainClass {
public static void main(String[] args)
{

    String[] months = {"January","February","March","April","May","June","July","August","September","October","December"};

    System.out.println(Arrays.toString(months)); //printing before


    //neither this works:
    Arrays.sort(months, 
            (a, b) -> Integer.signum(a.length() - b.length())   
    );

    //nor this:
    Arrays.sort(months, 
            (String a, String b) -> { return Integer.signum(a.length() - b.length()) }; 
    );


    System.out.println(Arrays.toString(months)); //printing after
}
}
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2
  • "Did it exactly the same way, yet it doesn't work." No you didn't, read carefully what they did. You have syntax errors. Commented Feb 23, 2014 at 16:11
  • @ZouZou Perhaps he did. I don't think the OP is even using Java 8. Commented Feb 23, 2014 at 16:21

4 Answers 4

Reset to default
89

The cleanest way would be:

Arrays.sort(months, Comparator.comparingInt(String::length));

or, with a static import:

Arrays.sort(months, comparingInt(String::length));

However, this would work too but is more verbose:

Arrays.sort(months,
            (String a, String b) -> a.length() - b.length());

Or shorter:

Arrays.sort(months, (a, b) -> a.length() - b.length());

Finally your last one:

Arrays.sort(months, 
    (String a, String b) -> { return Integer.signum(a.length() - b.length()) }; 
);

has the ; misplaced - it should be:

Arrays.sort(months, 
    (String a, String b) -> { return Integer.signum(a.length() - b.length()); }
);
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11 Comments

+1 for showing Comparator.comparing, it is so much easier to grasp than writing a custom lambda or Comparator.
I suggest using Integer.compare instead of signum of -.
@PeterLawrey Indeed - signum(a-b) has the risk of overflowing (even if it's unlikely for strings).
@lavsprat huh.... Version 7 != Version 8... Java 8 is here: jdk8.java.net/download.html
I don't think - can overflow in this case, as both values are non-negative.
|
3

You're looking for this:

Arrays.sort(months, (a, b) -> Integer.signum(a.length() - b.length()));
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4 Comments

Works for me... Are you sure you're using Java 8? Which IDE are you using?
@lavsprat It also works for me, it prints [May, June, July, March, April, August, January, October, February, December, September]... What Java 8 version are you using?
I'm using Eclipse. This site says I have Version 7 Update 51.
Well of course you can't use lambdas then, as they are only available in Java 8.
2

The functionality you are looking for is in Java 8, which has not yet been released. It is scheduled for release in a few months if you want to wait, or if not beta downloads are available.

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4 Comments

Useful comment, but not an answer.
@TedHopp It is an answer. His code isn't compiling because he's trying to use a Java 8 feature in Java 7!
His code isn't compiling because it has a syntax error (semicolon at the end of the second argument to Arrays.sort).
This comment is a bit misleading, unless you count three weeks as being "a few months": ship date is March 18th
1

You can try this:

Arrays.sort(months, (a,b) -> Integer.compare(a.length(), b.length()));
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1 Comment

Note that Integer#compare doesn't state it, but the return value is always -1, 0, or 1 (whereas the javadoc implies it's merely <0 or >0, so it is subject to change). #signum explicitly states this relationship. Either way, for Arrays#sort, I don't see what this answer contributes that the others didn't already cover 9 years ago.

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